1. Field of the Invention
This invention relates to a process for drawing gel-spun polyethylene multi-filament yarns and the drawn yarns produced thereby. The drawn yarns are useful in armor, composites, fishing line, ropes, sutures, fabrics and other applications.
2. Description of the Related Art
Gel-spun polyethylene fibers are prepared from ultrahigh molecular weight polyethylene (UHMW PE). The preparation and drawing of UHMW PE filaments in the gel state was first described by P. Smith, P. J. Lemstra, B. Kalb and A. J. Pennings, Poly. Bull., 1, 731 (1979). Single filaments were spun from 2 wt. % solution in decalin, cooled to a gel state and then stretched while evaporating the decalin in a hot air oven at 100 to 140° C.
Further descriptions of the drawing of polyethylene filaments containing substantial concentrations of solvent such as decalin or wax were described for example in P. Smith and P. J. Lemstra, Macromol. Chem., 180, 2983 (1979); J. Matl. Sci., 15, 505 (1980); and in GB 2,042,414A; U.S. Pat. No. 4,411,854; U.S. Pat. No. 4,422,993; U.S. Pat. No. 4,430,383; U.S. Pat. No. 4,436,689; EP 0 077,590; U.S. Pat. No. 4,617,233; U.S. Pat. Nos. 4,545,950; 4,612,148; U.S. Pat. No. 5,246,657; U.S. Pat. No. 5,342,567; EP 0 320,188 A2 and JP-A-60/5264.
Some of the problems associated with drawing of polyethylene in a gel state were discussed in Japan Kokai Patent Publication 238416-1995. If the drawing is conducted at a high temperature without prior removal of a solvent, melt fusion occurs and so it is necessary to lower the drawing temperature. However, at a lower temperature it is impossible to conduct the drawing to the high draw ratio necessary to obtain high strength.
The drawing of gel-spun high strength polyethylene filaments in essentially a diluent-free state was first described by B. Kalb and A. J. Pennings, Poly. Bull., 1, 871 (1979). Single filaments were spun from dodecane solution and simultaneously dried and stretched in a heated tube under an increasing temperature of 100 to 148° C. A dried filament of about 10 g/d tenacity was then re-stretched at 153° C. to a tenacity of about 29 g/d.
Further descriptions of the drawing of gel-spun polyethylene filaments in an essentially diluent-free state were described for example in B. Kalb and A. J. Pennings, Polymer, 21, 3 (1980); J. Smook et. al, Poly. Bull., 2, 775 (1980); P. Smith et el., J. Poly Sci., Poly Phys. Ed., 19, 877 (1981); J. Smook and A. J. Pennings, J. Appl. Poly. Sci., 27, 2209 (1982), J. Matl. Sci., 19, 31 (1984), J. Matl. Sci., 19, 3443 (1984); J. P. Penning et al., Poly. Bull., 31, 243 (1993); Japan Kokai Patent Publication 238416-1995; and in U.S. Pat. Nos. 4,413,110; 4,536, 536; 4,551,296; 4,663,101; 5,032,338; 5,286,435; 5,578,374; 5,736,244; 5,741,451; 5,958,582; 5,972,498; and 6,448,359 B1.
Each of these references represented an advance in the state of the art, however none suggests the process of this invention, and none satisfies all of the needs met by this invention.
There may be several motivations for drawing gel-spun polyethylene filaments and yarns. The end-use applications may require low filament denier or low yarn denier. Low filament deniers are difficult to produce in the gel spinning process. Solutions of UHMW PE are of high viscosity and may require excessive pressures to extrude through small spinneret openings. Hence, use of spinnerets with larger openings and subsequent drawing may be a preferable approach to producing fine denier filaments.
Another motivation for drawing may be a need for high tensile properties. Tensile properties of gel-spun polyethylene filaments generally improve with increased draw ratio if appropriately conducted.
Yet another motivation for drawing may be to produce a special microstructure in the filaments that may be especially favorable for particular properties, for example, ballistic resistance.
Each of these motivations must be balanced against the efficiency, productivity and cost of the drawing operation. Drawing of gel-spun polyethylene yarns requires the use of high horsepower motors and heavy rolls. Drawing speeds and productivity are limited by breakage of filaments. A need exists to identify drawing processes that permit the accomplishment of objectives of reducing denier and increasing yarn properties with enhanced efficiency, productivity and lower cost.